Microporous membranes are used in filtration and separation of protein, colloid, bacteria, virus, salts, etc. in various separation processes used in the arts of chemical industry, electronic industry, pharmaceutical industry, food industry, disposal of waste liquid, medical treatment (including medical instrument such as artificial organs, examination), water purification, etc. Porous materials such as porous bead and porous fiber are used as adsorbents for substances, e.g., solvent or protein, dissolved in water, adsorbents for gaseous substance, fillers for chromatography, carriers for enzyme in bioreactor or biosensor, etc.
As the process for the preparation of a microporous membrane (hereinafter occasionally referred simply to as "membrane") there has been most widely used a so-called wet process which comprises shaping a polymer solution, optionally drying somewhat the surface of said polymer solution, and then allowing said polymer solution to come in contact with a coagulating solution miscible with the solvent in said solution so that the polymer is coagulated in a network pattern. However, the wet process is disadvantageous in that only a solvent-soluble-polymer, i.e., linear polymer can be used. The resulting membrane exhibits a poor heat resistance, pressure resistance and fastness to filtration (compaction). Further, the membrane can be hardly modified, making it difficult to prepare a hydrophilic membrane. If it is tried to prepare a hydrophilic membrane for the purpose of inhibiting fouling (drop of flux due to fouling of membrane), which is the greatest weak point in membrane filtration, the resulting membrane swells in water and thus exhibits a poor compaction resistance. Further, the polymer material comprising a hydrophilic group incorporated therein can swell in water during the preparation of the membrane, making it possible to prepare a membrane having a small pore diameter. Moreover, the resulting membrane is alcohol-soluble and thus can be hardly purified. Accordingly, the preparation of a hydrophilic membrane requires complicated steps of preparing a non-hydrophilic membrane and then surface-coating the non-hydrophilic membrane to render it hydrophilic.
On the other hand, U.S. Pat. No. 5,236,588 discloses a process for the preparation of a microporous membrane which comprises shaping a mixture of an energy ray-polymerizable monomer and/or oligomer and a poor solvent which is miscible with said energy ray-polymerizable monomer and/or oligomer but doesn't dissolve or subject said energy ray-polymerizable monomer and/or oligomer thereinf to gelation, optionally drying somewhat the surface of the mixture, and then irradiating the material with energy ray to effect polymerization and phase separation at the same time. In accordance with this process, the use of a multifunctional compound as the monomer and/or oligomer makes it possible to prepare a crosslinked polymer membrane. Thus, a membrane having excellent heat resistance and compaction resistance can be obtained. Further, the addition of a hydrophilic material to the monomer and/or oligomer makes it easy to prepare a hydrophilic membrane. However, this process, too is disadvantageous in that if it is tried to prepare a hydrophilic membrane having a small cut-off molecular weight, only a membrane having a small flux can be obtained.
The foregoing difficulties are encountered in membranes as well as in porous materials such as porous bead and porous fiber.